The present invention relates to probe assemblies of the type commonly used for testing integrated circuits (ICs) that are fabricated on a wafer or substrate.
The trend in electronic production, particularly in integrated circuit technology, has been toward fabricating larger numbers of discrete circuit elements with higher operating frequencies and smaller geometries on a single substrate or “wafer.” After fabrication, the wafer is divided into a number of rectangular-shaped chips or “dies” where each die presents a rectangular or other regular arrangement of metallic bond or contact pads through which connections are made for the inputs and outputs of the electrical circuit on the die. Although each die is eventually packaged separately, for efficiency sake, testing of the circuits formed on the wafer is preferably performed while the dies are still joined together on the wafer. One typical procedure is to support the wafer on a flat stage or “chuck” and to move the wafer in X, Y and Z directions relative to the head of a probing assembly so that contacts on the probing assembly move relative to the surface of the wafer for consecutive engagement with the contact pads of one or more of a plurality of dies or test structures on the wafer. Respective signal, power and ground conductors that interconnect the test instrumentation with the contacts on the probing assembly enable each circuit on the wafer to be sequentially connected to the instrumentation and tested.
Gleason et al., U.S. Pat. No. 5,914,613, discloses a membrane probing system for use in a probe station. The membrane probing system comprises a probe head and a membrane probing assembly. The probe head includes an interface board, a multi-layer printed circuit board that facilitates interconnection of the membrane probing assembly and the test instrumentation supplying power and signals to and receiving signals from the electrical circuit being tested, the device-under test (DUT). The power and signals are transmitted over one or more conductors that are conductively interconnected with respective data/signal traces on the interface board. The data/signal traces on the interface board are conductively connected to respective conductive traces on the surface of the membrane assembly. A metallic layer below the surface of the interface board provides a ground plane for the interface board and a ground reference for the power and lower frequency signals.
Typically, higher frequency signals; commonly in the radio or microwave frequency ranges, collectively referred herein to as RF signals; are communicated between the test instrumentation and the membrane probing system with coaxial cable. The coaxial cable is connected to an adapter that is secured to the interface board. A second portion of coaxial cable, conductively interconnected with the first portion in the adapter, is connected to one or more conductive traces on the surface of the interface board. Typically, the end of the second portion of coaxial cable is cut at an angle and the conductors of the cable are connected to respective traces on the interface board to transition the signal path from the coaxial cable to a co-planar waveguide. For example, the center connector of the coaxial cable may soldered to a trace on the interface board while the outer conductor of the cable, connected to a ground potential, is soldered to a pair of traces that are respectively spaced apart to either side of the trace to which the center conductor is connected transitioning the signal path from coaxial cable to a ground-signal-ground (GSG) co-planar waveguide on the interface board. The traces on the interface board are conductively engaged with respective, corresponding traces on the lower surface of the membrane assembly extending the co-planar waveguide to the contacts on the membrane. The impedance of the transition signal path from the coaxial cable to the coplanar waveguide on the membrane is, ideally, optimized, with a typical value of 50 ohms (Ω). However, inconsistencies in connections with the ground plane of the interface board may cause the impedance of a particular signal path to vary from the desired matched impedance producing a reflection of the RF signals that are absorbed by other structures resulting in erratic performance of the probing system.
The membrane of the probing system is supported by a support element that is made of an incompressible material, such as a hard polymer, and detachably affixed to the upper surface of the interface board. The support element includes a forward support or plunger portion that protrudes though a central aperture in the interface board to project below the interface board. The forward support has the shape of a truncated pyramid with a flat forward support surface. The membrane assembly which is also detachably secured to the interface board by the support element includes a center portion that extends over and is separated from the forward support surface of the support element by an intervening elastomeric layer. The flexible membrane assembly comprises one or more plies of insulating sheeting, such as polyimide film. Flexible conductive layers or strips are provided between or on these layers to form power/data/signal traces that interconnect with the traces on the interface board at one end. The second end of the traces on the membrane terminate in conductive connections to respective contacts which are arranged on the lower surface of the portion of the membrane extending over the forward support. The contacts are arranged in a pattern suitable for contacting the bond pads of the DUT when the chuck is moved to bring the contacts of the probe assembly into pressing engagement with the bond pads.
The contacts of the probing system comprise a beam which is affixed to the lower surface of the membrane assembly and which is conductively interconnected with the appropriate trace on the surface of the membrane. A contact bump or tip for engaging a bond pad of the DUT is affixed to one end of the beam. When the contact bump is pressed against the bond pad of the DUT, the membrane assembly is deflected, compressing a portion of the elastomeric layer proximate the end of the beam to which the contact bump is affixed. The compliance of the elastomeric layer enables relative displacement of the respective contact bumps and facilitates simultaneous engagement with a plurality of bond pads that may have respective contact surfaces that lie in different planes. The resilience of the elastomeric layer controls the force exerted by the contacts and returns the contacts to the at-rest position when the probe is withdrawn from pressing engagement with the DUT.
The bond pads on DUTs are subject to the rapid development of a layer of oxidation which can electrically insulate the bond pad from the contact. To improve the conductivity of the bond pad/contact interface, the contacts of membrane probes are commonly pressed into the bond pad with sufficient force to penetrate the oxide layer. While penetration of the oxide layer improves conductivity, excessive force can damage the bond pad. With the membrane probe disclosed by Gleason et al, the force of contact with the bond pad is exerted at one end of the beam and the off-center loading on the beam causes the beam to rotate as the portion elastomeric layer adjacent the deflected end of the beam is compressed. The rotation of the beam causes the surface of the contact bump to translate across the bond pad surface and abrade or scrub the oxide coating on the surface improving conductivity between the bond pad and the contact.
However, the conductors within the membrane assembly and attached to the contacts can be broken by excessive displacement of the contacts or may fail from fatigue due to repeated bending when the contacts are displaced during probing. In addition, bond pad material may build up in the area of the contacts of wafer probing assemblies requiring frequent cleaning and, eventual replacement due to wear. While the membrane assembly is detachable from the interface card for cleaning or replacement, the membrane assembly is complex and fairly expensive to replace.
What is desired, therefore, is a probing apparatus having improved impedance characteristics, longer service life and less expensive contacts that can be quickly replaced.